Process for the thermal stabilization of polyacrylonitrile fibers andfilms

ABSTRACT

An improved process for the thermal stabilization of an acrylic fibrous material or film is provided. The fibrous precursor or film is impregnated with 0.5 to 10 per cent stannous chloride by contact with a solution of the same provided at a moderate temperature, dried to remove the solvent, and heated in a gaseous atmosphere containing 30 to 100 per cent by weight molecular oxygen at a more highly elevated temperature until a stabilized fibrous material or film is formed. The presence of the stannous chloride in combination with the gaseous atmosphere containing more than the usual concentration of molecular oxygen (e.g. 10 to about 20 per cent by weight) has been found to result in a substantially improved process. More specifically, the resulting stabilized acrylic fibrous materials and films exhibit enhanced physical properties (i.e., strength and modulus), and the stabilization reaction is accelerated in a controlled manner in the substantial absence of undesirable fiber coalescence. The resulting stabilized fibrous material or film is non-burning, and may be utilized as a fire resistant fiber, fabric, or film, or optionally carbonized or carbonized and graphitized to form a carbonaceous fibrous material or film.

United States Patent [191 1 Di Edwardo et al.

[ 1 PROCESS FOR THE THERMAL STABILIZATION OF I POLYACRYLONITRILE FIBERSAND FILMS [75] Inventors: Andrew H. Di Edwardo, Parsippany;

' Klaus H. Gump, Gillette, both of [73] Assignee: Celanese Corporation,New York,

22 Filed: Apr. 28, 1972 211, App]. No.: 248,372

France 423/447 [111 3,820,951 [451 June 28,1974

Primary Examiner-Leon D. Rosdol Assistant Examiner-l-larold Wolman [571ABSTRACT An improved process for the thermal stabilization of an acrylicfibrous material or film is provided. The fibrous precursor or film isimpregnated with 0.5 to 10 per cent stannous chloride by contact with asolution of the same provided at a moderate temperature, dried to removethe solvent, and heated in a gaseous atmosphere containing 30 to 100 percent by weight molecular oxygen at a more-highly elevated temperatureuntil a stabilized fibrous material or film is formed. The presence ofthe stannous chloride in combination with the gaseous atmospherecontaining more than the usual concentration of molecular oxygen (eg 10to about 20 per cent by weight) has been found to result in asubstantially improved process.

More specifically, the resulting stabilized acrylic fibrous materialsand films exhibit enhanced physical properties (i.e. strength andmodulus), and the stabilization reaction is accelerated in a controlledmanner in the substantial absence of undesirable fiber coalescence. Theresulting stabilized fibrous material or film is non-burning, and may beutilized as a fire resistant fiber, fabric, or film, or optionallycarbonized or carbonized andgraphitized to form a carbonaceous fibrousmaterial or film.

16 Claims, No Drawings BACKGROUND OF THE INVENTION In the pastprocedures have been proposed for the conversion of fibers formed fromacrylic polymers to a modified form possessing enhanced thermalstability. Such modification has generally be accomplished by heatingfibrous material in an oxygen-containing atmosphere at a moderatetemperature for an extended period of time.

U.S. Pat. Nos. 2,913,802 to Barnett and 3,285,696 to Tsunoda discloseprocesses for the conversion of fibers of acrylonitrile homopolymers orcopolymers to a heat resistant form. The stabilization of fibers ofacrylonitrile homopolymers and copolymers in an oxygencontainingatmosphere involves (l) a chain scission and oxidative cross-linkingreaction of adjoining molecules, (2) dehydrogenation reactions,.as wellas (3) a cyclization reaction of pendant nitrile groups. It is generallyrecognized that the rate at which the stabilization reaction takes placeincreases with the temperature of theoxygen-containing atmosphere.However, the stabilization reaction must by necessity be conducted atrelatively low temperatures (i.e., below about 300 C.), since thecyclization reaction is exothermic in nature and must be controlled ifthe original fibrous configuration of the material undergoingstabilization is to be preserved. Accordingly the stabilization reactiontends to be time consuming, and economically demanding because of lowproductivity necessitated by the excessive time requirements. Priorprocesses proposed to shorten the period required by the stabilizationreaction include that disclosed in U.S. Pat. No. 3,416,874. See also theprocesses of commonly assigned U.S. Pat. Ser. Nos. 777,901, filed Nov.21, 1968, of K. H. Gump and D. E. Stuetz (now U.S. Pat. No. 3,592,595)wherein the cyclization of pendant nitrile groups of the acrylic fibrousmaterial is catalytically enhanced while the fibrous material isimmersed in a solution of a Lewis acid at a temperature of about 160 C.;U.S. Pat. No. 109,672, filed Jan. 25, 1971, of E. C. Chenevey and R. M.Kimmel (now U.S. Pat. No. 3,708,326); and U.S. Pat. No. 200,183, filedNov. 18, 1971, of K. H. Gump and DE. Stuetz. i

U.S. Pat. No. 3,242,000 to J. A. Lynch discloses an unrelated processfor producing carbonized textile products from acrylic textile productswherein a refractory metal oxide barrier coating is formed upon thesurface of fabrics employing a heat treatment atmosphere which containsat least some oxygen(e.'g. about to about 20 per centoxygen).

While stabilized acrylic fibrous materials may be used directly inapplications where a non-buming fiber is required, demands for the samehave been increasingly presented by manufacturers of carbonized fibrousmaterials. Carbonized fibrous materials are commonly formed by heating astabilized acrylic fibrous material in an inert atmosphere, such asnitrogen or argon, at a more highly elevated temperature. During thecarbonization reaction elements such as nitrogen, oxygen and hydrogenare substantially expelled. Accordingly, the term carbonized as used inthe art commonly designates a material consisting of at least about 90per cent carbon by weight, and generally at least about 95 per centcarbon by weight. Depending upon the conditions under which a carbonizedfibrous material is processed, it may or may not contain graphiticcarbon as determined by the characteristic x-ray diffraction pattern ofgraphite. See, for instance, commonly assigned U.S.

5 Pat. Ser. No. 777,275, filed Nov. 20, 1968, of Charles M. Clarke (nowabandoned) for a preferred procedure for forming carbonized andgraphitized fibrous materials from a stabilized acrylic fibrousmaterial.

It is an object of the invention to provide an improved process forenhancing the thermal stability of an acrylic shaped article.

It is an object of the invention to provide an improved process for theflame-proofing of a fibrous material or film formed from acrylicpolymers.

It is an object of the invention to provide an improved process whereinthe thermal stabilization of an acrylic fibrous material or film isaccelerated in a controlled and non-deleterious manner.

It is an object of the invention to provide an improved process for thethermal stabilization of an acrylic fibrous material or film whichproduces a stabilized product exhibiting enhanced physical properties,i.e., strength and modulus.

It is an object of the invention to provide an improved process for thestabilization of acrylic fibers which is carried out in an enrichedoxygen-containing atmosphere on an expeditious basis in the absence ofexpected fiber coalescence.

It is another object of the invention to provide an improved process forthe stabilization of fibrous materials or films formed from acrylicpolymers which results in a superior product which is suitable forcarbonization,

or carbonization and graphitization.

It is a further object of the invention to provide a process forconverting a fibrous acrylic material or film to a stabilized formpossessing substantially the identical configuration as the startingmaterial. a These and other objects, as well as the scope, nature andutilization of the invention will be apparent from the following 1detailed description and appended claims.

SUMMARY or THE INVENTION It has been found that an improved process forthe stabilization of an acrylic fibrous material or film selected fromthe group consisting of an acrylonitrile homopolymer and acrylonitrilecopolymers containing at least about mol per cent of acrylonitrile unitsand up to about 15 mol per cent of one or more monovinyl unitscopolymerized therewith to produce a stabilized product exhibitingenhanced mechanical properties comprises:

(a) impregnating the fibrous material or film with about 0.5 to 10 percent by weight of stannous chloride by contact with a solution of thestannous chloride in a solvent incapable of dissolving the fibrousmaterial or film provided at a temperature of about 0 to C. whilepreserving the original configuration of the fibrous material or filmsubstantially intact, b. drying said fibrous material or film tosubstantially remove the solvent therefrom, and c. heating the resultingimpregnated and dried fibrous material or film in a gaseous atmospherecontaining 30 to 100 per cent by weight molecular oxygen provided at atemperature of about 260 to 350 C. until a stabilized fibrous materialor film is DESCRIPTION OF PREFERRED EMBODIMENTS The acrylic shapedarticles, i.e., fibers or films, undergoing stabilization in the presentprocess may be formed by conventional solution spinning techniques(i.e., may be dry spun or wet spun) or by conventional solvent castingtechniques, and are commonly drawn to increase their orientation. As isknown in the art, dry spinning is commonly conducted by dissolving thepolymer in an appropriate solvent, such as N,N-dimethylformamide orN,N-dimethylacetamide, and passing the solution through an opening ofpredetermined shape into an evaporative atmosphere (e.g. a nitrogen) inwhich much of the solvent is evaporated. Wet spinning is commonlyconducted by passing a solution of the polymer through an opening ofpredetermined shape into an aqueous coagulation bath. Casting iscommonly conducted by placing a solution containing the polymer upon asupport, and evaporating the solvent therefrom.

The acrylic polymer utilized as the starting material is formedprimarily of recurring acrylonitrile units. For instance, the acrylicpolymer should generally contain not less than 'about.85 mol per cent ofacrylonitrile units and not more than about 15 mol per cent of unitsderived from a monovinyl compound which is copolymerizable withacrylonitrile such as stryene, methyl acrylate, methyl methacrylate,vinyl acetate, vinyl chloride, vinylidene chloride, vinyl pyridine, andthe like, or a plurality of such monomers.

The preferred acrylic precursor is an acrylonitrile homopolymer.Preferred acrylonitrile copolymers contain at least about 95 mol percent of acrylonitrile units and up to about mol per cent of one or moremonovinyl units copolymerized therewith.

The acrylic precursor is preferably provided as a continuous length of afibrous material and may be in a variety of physical configurations. Forinstance, the acrylic fibrous materials may be present in the form ofcontinuous lengths of multifilament yarns, tows, tapes, strands, cables,or similar fibrous assemblages. Altematively, acrylic films ofrelatively thin thickness, e.g. about 1 to mils, may be selected as theprecursor.

When the starting material is a continuous multifilament yarn, a twistmay be imparted to the same to improve the handling characteristics. Forinstance, a twist of about 0.1 to 5 tpi, and preferably about 0.3 to 1.0tpi may be utilized. Also a false twist may be used instead of or inaddition to a real twist. Alternatively, one may select bundles offibrous material which possess substantially no twist.

The starting material may be drawn in accordance with conventionaltechniques in order to improve its orientation. For instance, thestarting material may be drawn by stretching while in contact with a hotshoe at a temperature of about 140 to 160 C. Additional representativedrawing techniques are disclosed in U.S. Pat. Nos. 2,455,173; 2,948,581;and 3,122,412. It is recommended that the acrylic fibrous materialsselected for use in the process be drawn to a single filament tenacityof at least about 3 grams per denier. If desired, however, the startingmaterial may be more highly oriented, e.g. drawn up to a single filamenttenacity of about 7.5 to 8 grams per denier, or more. Acrylic filmsoptionally may be either uniaxially or biaxially oriented.

Prior to heating the acrylic fibrous material or film in the gaseousatmosphere of enriched molecular oxygen content to accomplish thedesired stabilization (as described hereafter), the precursor isimpregnated with 0.5 to 10 per cent by weight of stannous chloride(preferably l to 5 per cent by weight) by contact with a solution of thesame in a solvent incapable of dissolving the fibrous material or film,and is dried to substantially remove the solvent used in the formationof the solution. The nature of the solvent selected may be widelyvaried. Particularly preferred solvents are'water, methano], andmixtures of methanol and water. It is essential that the solvent havethe ability to dissolve the stannous chloride while being incapable ofdissolving or otherwise adversely influencing the acrylic materialundergoing treatment. Other representative solvents for the stannouschloride and non-solvents for the acrylic fibrous material or filminclude acetonitrile, tricresylphosphate, and phenylether. Solvents mayalso be selected which have a tendency to swell the fiber such asmixtures of dimethylformamide and water. The stannous chloride commonlyis dissolved in the solvent in a concentration of about 1 to 10 per centby weight based upon the total weight of the solution. When contactedwith the acrylic fibrous material or film, the stannous chloridesolution is provided at a temperature of about 0 to C., and preferablyat a temperature of about 10 to 40 C. Relatively brief contact times,e.g. 3 seconds to 5 minutes, are generally adequate. Contact timesdepend primarily upon the concentration of stannous chloride in thesolution, and relative freedom of access of the solution throughout theacrylic material undergoing impregnation. If a fibrous material isprovided as a relatively compact assemblage, longer contact times arerequired. For instance, when an acrylic fibrous material is immersed inan aqueous solution containing 2 per cent stannous chloride based uponthe total weight of the solution provided at 25 C. for 5 seconds underconditions wherein ready access is possible, the stannous chlorideuptake following evaporation of the solvent is commonly about 9 to 12per cent by weight. The stannous chloride which is introduced into thesolvent to form the solution of same may be either anhydrous [e.g. SnClor hydrous [e.g. SnCl '2l-l O]. When stannous chloride is present in thehydrous form, the water of hydration is not included when calculatingthe concentration of stannous chloride for the purposes of the presentspecification and appended claims.

The impregnation step of the process may be conducted on either a batchor a continuous basis, and preferably while the fibrous material ismaintained at a substantially constant length in the absence ofappreciable shrinkage. For instance, a continuous length of the acrylicprecursor may be wound upon a mandrel or other support and immersed inthe solution containing the stannous chloride, or continuously passedthrough the same, e.g. in the direction of its length while guided byrollers or other guide means. Contact between the acrylic material andthe solution may alternatively be made by spraying or other paddingtechnique as will be apparent to those skilled in the art. Highersolution temperatures than about 100 C. are to be avoided'in order todiminish the possibility of the premature trile groups at the surface.

Following impregnation the acrylic fibrous material or film is nextdried to that the solvent is substantially removed, and the requiredquantity of stannous chloride provided in intimate associationtherewith. The drying step may be conducted in any convenient manner.The impregnated acrylic precursor may be simply exposed to ambientconditions until solvent adhering thereto is substantially evaporated.For instance, drying may be conducted by exposure to a gaseousatmosphere (eg. air) at a temperature of about to 40 C; The drying stepcan, of course, be expedited by exposure to a circulating gaseousatmosphere at a more highly elevated temperature, or even in the samezone where the stabilization reaction is carried out (as describedhereafter). It is recommended, however, that drying be conducted at amoderate temperature below about 100 C. because of the possibility ofadversely influencing the tensile properties of the acrylic materialduring the vigorous evolution of solvent at a more highly elevatedtemperature. The resulting impregdated and dried acrylic materialcontains about 0.5 to 10 per cent by weight of stannous chloride, andpreferably about 1 to 5 per cent by weight of stannous chloride.

The resulting impregnated and dried acrylic material is heated in agaseous atmosphere containing 30 to 100 per cent by weight molecularoxygen provided at a temperature of about 260 to 350- C. until astabilized fibrous product or film is formed which retains its originalconfiguration substantially intact and which is nonburning whensubjected to an ordinary match flame. The portion of the gaseousatmosphere other than molecular oxygen, if any, is preferablysubstantially unreactive with the acrylic fibrous material during thestabilization treatment, eg it may include nitrogen, hydrogen, carbondioxide, carbon monoxide, argon, helium, etc. In a preferred embodimentof the process, the oxygen-containing atmosphere is air enriched withmolecular oxygen. Molecular oxygen is preferably presentin the gaseousatmosphere in a concentration of 35 to 100 per cent by weight, and mostpreferably in a concentration of about 40 to 60 per cent by weight.Preferred temperatures for the oygen-containing atmosphere range fromabout 290 to 310 C. If desired, the fibrous material or film may beexposed to a temperature gradient wherein the temperature isprogressively increased. The presence of an enriched oxygen atmospherein combination with the presence of stannous chloride has been found tobe of prime importance in accomplishing the improved stabilizationresults discussed hereafter. t For best results during the stabilizationreaction uniform contact with the gaseous atmosphere throughout allportionsof the impregnated acrylic material is encouraged. Suchuniformreaction conditions can best be accomplished by limiting the mass offibrous mate rial or film at any one location so that heat dissipationfrom within the interior of the same is not unduly impaired, and freeaccess to molecular oxygen is provided. For instance, the acrylicfibrous material or. film may be placed in the gaseous atmosphere whilewound upon a support to a limited thickness. in a preferred embodimentof the invention, the stannous chloride impregnated acrylic fibrousmaterial or film is continuously passed in the direction of its lengththrough the heated gaseous atmosphere. For instance, a continuous lengthof the acrylic fibrous material or film may be passed through acirculating oven or the tube of a muffle furnace. The speed of passagethrough the heated oxygen-containing atmosphere will be determined bythe size of the heating zone and the desired residence time. i

The period of time required to complete the stabilization reactionwithinthe gaseous atmosphere is generally inversely related to thetemperature of the atmosphere, and is also influenced by the denier ofthe acrylic fibrous material or the thickness of the film undergoingtreatment, and the concentration of molecular oxygen in the atmosphere.Treatment times in the oxygen-containing atmosphere accordingly commonlyrange from about 6 minutes to minutes.

The stabilized acrylic fibrous materials or films formed in accordancewith the present process are black in appearance, retain substantiallythe sameconfiguration as the starting material, are non-burning whensubjected to an ordinary match flame, commonly have a bound oxygencontent of at: least 7 (eg 7 to 12) per cent by weight as determined bythe Unterzaucher, or other suitable analysis, commonly contain fromabout 50 to per cent carbon by weight, and commonly contain about 0.4 to8 per cent tin by weight.

The theory whereby the presence of stannous chloride in combination witha greater than usual oxygen concentration in the gaseous atmosphereproduces improved stabilization results is considered complex andincapable of simple explanation. The results achieved are considered tobe surprising and unexpected. While it has been suggested in the pastthat acrylic stabilization reactions can be conducted in an atmosphereof air enriched with oxygen, the results of such stabilizationconditions have tended to'be less than optimum particularly ifrelatively high stabilization temperatures (e.g. 260 C. and above) areselected because of the increased tendency for an explosive ,exoth'ermto occur under such conditions. Such an exothermic reaction at the veryleast produces a weak and brittle product, and may result in a completebreakage of the acrylic fiber or fragmentation of the acrylic film. Ithas now been found after extensive experimentation that while stannouschloride has the ability to accelerate the kinetics of the cyclizationportion of the stabilization reaction in air, that the presence of thiscompound has the concomitant tendency to retard the dehydrogenation andoxidative cross-linking portions of the stabilization reaction.Additionally, even when the oxygen concentration of the gaseousstabilization atmosphere is increased, the mechanical properties of theresulting product are surprisingly not diminished, and even moresurprisingly are enhanced. For instance, stabilized products formedinthe present process in the presence of stannous chloride exhibit ahigher tenacity and modulus than if produced in the presence of stannouschloride in'air. Not only is the stabilization reaction accelerated, butno substantial fiber coalescence occurs and less fiber weight lossresults. The process of the present invention proceeds at an expeditiousrate in a .controlled fashion with the fiber temperature during thestabilization reaction more closely approximating that of the gaseousatmosphere while eliminating a delterious exothermic reaction.

In our commonly assigned US. Pat. Ser; No.

248,371, filed concurrently herewith, is disclosed a related processwherein stannous chloride is dissolved within a solution of the acrylicpolymer, the solution formed into a fiber or film which contains thestannous chloride in intimate association therewith, and the resultingfiber or film stabilized in a gaseous atmosphere containing more thanthe usual concentration of oxygen.

The stabilized fibrous material resulting from the stabilizationtreatment of the present process is suitable for use in applicationswhere a fire resistant fibrous material is required. For instance,non-burning fabrics may be formed from the same. As previouslyindicated, the stabilized acrylic fibrous materials are particularlysuited for use as intermediates in the production of carbonized fibrousmaterials. Such amorphous carbon or graphitic carbon fibrous productsmay be incorporated in a binder or matrix and serve as a reinforcingmedium. The carbon fibers may accordingly serve as a lightweight loadbearing component in high perform ance composite structures which findparticular utility in the aerospace industry.

The stabilized film resulting from the stabilization treatment issuitable for use in applications where a fire resistant sheet materialis required. Such stabilized films may be also utilized as intermediatesin the production of carbonized films. Carbonized films may be utilizedin the formation of lightweight high temperature resistant laminateswhen incorporated in a matrix material (e.g. an epoxy resin).

The following examples are given as specific illustrations of theinvention. It should be understood, however, that the invention is notlimited to the specific details set forth in the examples.

EXAMPLE I A continuous length of an 800 fil dry spun acrylonitrilehomopolymer continuous filament yarn having a total denier of about 960is selected as the starting material. The yarn exhibits a twist of 0.5tpi and has been drawn to a single filament tenacity of 2 grams perdemer.

The yarn is immersed for 5 seconds in a 5 per cent by weight solution ofstannous chloride in methanol by use of a continuous padding apparatuswherein the yarn is maintained at a constant length through theadjustment of the longitudinal tension thereon. The solution is providedat a temperature of about 25 C. while the acrylonitrile homopolymer yarnis immersed therein. While immersed in the solution, the yarn becomesimpregnated with about 5 per cent by weight of stannous chloride.

The stannous chloride impregnated yarn upon removal from the solution isdried while at constant length by contact with circulating air in atubular furnace through which it is passed for 2 minutes wherein themethanol solvent is substantially removed. The temperature within thetubular drying zone is 80 C.

A portion of the yarn containing 5 per cent by weight stannous chlorideis next stabilized .on a continuous basis by heating in a circulatinggaseous atmosphere of air enriched with molecular oxygen provided in amuffle furnace. The total oxygen concentration in the atmosphere is 40per cent by weight. The gaseous atmo- 8 sphere is provided at atemperature of 290 C. and the residence time therein is 18 minutes. Theyarn is maintained under a longitudinal tension sufiicient to maintain asubstantially constant length during the stabilization reaction.

The resulting stabilized yarn is black in appearance, non-brittle,flexible, has a textile-like hand, retains its original fibrousconfiguration substantially intact, is non-burning when subjected to anordinary match flame, retains strength after glowing in a match flame,and'has an oxygen content in excess of 8 per cent by weight asdetermined by the Unterzaucher analysis.

In a control run an identical sample of the acrylonitrile homopolymeryarn is passed through the muffle furnace in an identical manner withthe exception that it has not been previously impregnated with stannouschloride. The resulting yarn is coalesced, extremely brittle, and nonflexible.-

The resulting stabilized yarn of Example I is carbonized and graphitizedin accordance with the teachings of [1.8. Pat. Ser. No. 777,275, filedNov. 20, 1968, of Charles M. Clarke (now abandoned) which is hereinincorporated by reference. The graphite yarn exhibits satisfactorytensile properties.

EXAMPLE ll Example I is repeated with the exception that portions of thestannous chloride impregnated yarn are stabilized for 10 minutes in airenriched with molecular oxygen atmospheres containing 40 and percent byweight molecular oxygen by weight provided at 300 C.

The stabilized yarn produced in the 80 per cent by weight molecularoxygen gaseous atmosphere possesses a single filament tenacity of 2.09grams per denier, and a Youngs modulus of 75.2 grams per denier.

For comparative purposes the process of Example [I is repeated with theexception that the gaseous atmosphere is air only and contains 20.9 percent by weight molecular oxygen. The resulting fibers burn when subjected to an ordinary match flame, are visibly coalesced, exhibit asingle filament tenacity of only 0.92 grams per denier, and a singlefilament Youngs modulus of only 47.5 grams per denier.

EXAMPLE III A rectangular section of acrylonitrile homopolymer filmhaving a thickness of 5 mils is selected as the starting material. Thefilm is impregnated stannous chlorde by immersion for 10 seconds in a 5per cent by weight solution of stannous chloride in methanol. Thesolution is provided at a temperature of about 25 C. while theacrylonitrile homopolymer film is immersed therein.

While immersed in the solution the film becomes impregnated with about 3per cent by weight of stannous chloride.

, The stannous chloride impregnated film upon removal from the solutionis dried by placement in a circulating air oven for 30 minutes whereinthe methanol solvent is substantially removed. The temperature withinthe drying zone is 80 C. The resulting impregnated and dried film isnext suspended for 8 minutes in an oven provided with a circulating airenriched with molecularoxygen atmosphere containing 40 per cent byweight molecular oxygen and maintained at 300 C. wherein it is convertedto a stabilized form while retaining its original configurationsubstantially intact. The resulting stabilized film is black inappearance, nonbrittle, flexible, non-bumin g when subjected to anordinary match flame, and contains a bound oxygen content in-excess ofabout 7 per cent by weight as determined by the Unterzaucher analysis.

In a Control run an identical sample of the acrylonitrile homopolymerfilm is heated in the oven in an identical manner with the exceptionthat it has not been impregnated with stannous chloride. The resultingfilm is brittle and without any physical strength.

Although the invention has been described with preferred embodiments,;itis to be understood that variations and modifications may be resorted toas will be apparentto those skilled in the art. Such variations and.

modifications are to be considered within the purview and scope of theclaims appended hereto.

We claim:

1. An improved process for the thermal stabilization of an acrylicfibrous material or film-selected from the groupconsisting of anacrylonitrile homopolymer and acrylonitrile copolymers containing atleast about 85 mol per cent of acrylonitrile units and up to about 15mol per cent of one or more monovinyl units copolymerized therewith toproduce a thermally stabilized product exhibiting enhanced physicalproperties com prising: t

a. impregnating said fibrous material or film with about 0.5 to l percent by weight of stannous chloride based upon the weight of saidacrylic fibrous material or film by contact with a solution of saidstannous chloride in a solvent incapable of dissolving said fibrousmaterial or film provided at a temperature of about 0 to 100 C. whilepreserving the original configuration of said fibrous material orfilmsubstantially intact,

b. drying said fibrous material or film to substantially remove saidsolvent therefrom, and

c. heating said resulting impregnated and dried fibrous material or filmin a gaseous atmosphere containing 30 to 100 per cent by weightmolecular oxygen provided at a temperature of about 260 to 350 C. untila thermally stabilized fibrous material or film is formed which is blackin appearance, retains its original configuration substantially intact,contains a boundoxygen content to at least 7 per cent by weight, andwhich is non-burning when subjected to an ordinary match flame, with anyportion of said gaseous atmosphere other than molecular oxygen beingsubstantially unreactive with the material undergoing stabilization.

2. An improved process of'claim l in which the precursor is a fibrousmaterial.

3. An improved process of claim 1 in which the cursor is a film.

4. An improved process of claim 2 in which said acrylic fibrous materialis an acrylonitrile homopolymer.

5. An improved process of claim 2 in which said acrylic fibrous materialis an acrylonitrile copolymer containing at least about mol per cent ofacrylonitrile units and up to about 5 mol per cent of one or moremonovinyl units copolymeriized therewith.

6. An improved process of claim 1 in which said solution of saidstannous chloride is provided at a temperature of about 10 to 40 C.during said impregnation step (a). t

7. An improved process of claim 1 in which said sol vent for saidstannous chloride is water.

8. An improved process of claim 1 in which said solvent for saidstannous chloride is methanol.

9. An improved process of claim 1 in which said drying step (b) isconducted at a temperature of about 10 to 40 C.

pre-

10. An improved process of claim 1 in which said resulting impregnatedand dried fibrous material or film contains said stannous chloride in aconcentration of about 1 to 5 percent by weight based upon the weight ofsaid acrylic. fibrous material or film immediately prior to heating insaid gaseous atmosphere containing 30 to. l00 per cent molecular oxygenby weight.

11. An improved process of claim 1 in which said gaseous atmospherecontains about35 to per cent mo: lecular oxygen by weight.

12. An improved process for the thermal stabilization of an acrylicfibrous material selected from the group consisting of an acrylonitrilehomopolymer and acrylonitrile copolymers containing at least 85' mol percent of acrylonitrile units and upto about 15 mol per cent of one ormore monovinyl units copolymerized therewith to produce atherrnallystabilized product exhibiting enhanced physical'propertiescomprising:

a. impregnating said fibrous material with about I to 5 per cent byweight of stannous chloride based upon the weight of said acrylicfibrous material by contact with a solution of said stannous chloridewherein the solvent is selected from the group consisting of water,methanol, and a mixture of methanol and water with the solution beingprovided at a temperature of about 10 to 40 C.,

drying said fibrous material to substantially remove the solventtherefrom, and c. heating said resulting impregnated and dried fibrousmaterial in a gaseous atmosphere containing 35 to 100 per cent by weightmolecular oxygen 12 drying step (b) is conducted at a temperature ofabout 10 to 40 C.

16. An improved process of claim 12 in which said gaseous atmospherecontains about 40 to 60 per cent molecular oxygen by weight.

2. An improved process of claim 1 in which the precursor is a fibrousmaterial.
 3. An improved process of claim 1 in which the precursor is afilm.
 4. An improved process of claim 2 in which said acrylic fibrousmaterial is an acrylonitrile homopolymer.
 5. An improved process ofclaim 2 in which said acrylic fibrous material is an acrylonitrilecopolymer containing at least about 95 mol per cent of acrylonitrileunits and up to about 5 mol per cent of one or more monovinyl unitscopolymerized therewith.
 6. An improved process of claim 1 in which saidsolution of said stannous chloride is provided at a temperature of about10* to 40* C. during said impregnation step (a).
 7. An improved processof claim 1 in which said solvent for said stannous chloride is water. 8.An improved process of claim 1 in which said solvent for said stannouschloride is methanol.
 9. An improved process of claim 1 in which saiddrying step (b) is conducted at a temperature of about 10* to 40* C. 10.An improved process of claim 1 in which said resulting impregnated anddried fibrous material or film contains said stannous chloride in aconcentration of about 1 to 5 per cent by weight based upon the weightof said acrylic fibrous material or film immediately prior to heating insaid gaseous atmosphere containing 30 to 100 per cent molecular oxygenby weight.
 11. An improved process of claim 1 in which said gaseousatmosphere contains about 35 to 100 per cent molecular oxygen by weight.12. An improved process for the thermal stabilization of an acrylicfibrous material selected from the group consisting of an acrylonitrilehomopolymer and acrylonitrile copolymers containing at least 85 mol percent of acrylonitrile units and up to about 15 mol per cent of one ormore monovinyl units copolymerized therewith to produce a thermallystabilized product exhibiting enhanced physical properties comprising:a. impregnating said fibrous material with about 1 to 5 per cent byweight of stannous chloride based upon the weight of said acrylicfibrous material by contact with a solution of said stannous chloridewherein the solvent is selected from the group consisting of water,methanol, and a mixture of methanol and water with the solution beingprovided at a temperature of about 10* to 40* C., b. drying said fibrousmaterial to substantially remove the solvent therefrom, and c. heatingsaid resulting impregnated and dried fibrous material in a gaseousatmosphere containing 35 to 100 per cent by weight molecular oxygenprovided at a temperature of about 290* to 310* C. while maintaining asubstantially constant length until a thermally stabilized fibrousmaterial is formed which is black in appearance, retains its originalconfIguration substantially intact, contains a bound oxygen content ofat least 7 per cent by weight, and which is non-burning when subjectedto an ordinary match flame, with any portion of said gaseous atmosphereother than molecular oxygen being substantially unreactive with thematerial undergoing stabilization.
 13. An improved process of claim 12in which said acrylic fibrous material is an acrylonitrile homopolymer.14. An improved process of claim 12 in which said acrylic fibrousmaterial is an acrylonitrile copolymer containing at least about 95 molper cent of acrylonitrile units and up to about 5 mol per cent of one ormore monovinyl units copolymerized therewith.
 15. An improved process ofclaim 12 in which said drying step (b) is conducted at a temperature ofabout 10* to 40* C.
 16. An improved process of claim 12 in which saidgaseous atmosphere contains about 40 to 60 per cent molecular oxygen byweight.